1. Field of the Invention
The present invention relates to a toner to be used in an image forming method including at least: a developing step of developing an electrostatic latent image formed on an electrostatic latent image-bearing member such as an electrophotographic photosensitive member or an electrostatic recording derivative in an electrophotographic method with a developer to form a toner image on the electrostatic latent image-bearing member; a transferring step of electrostatically transferring the toner image formed on the electrostatic latent image-bearing member onto a recording material through or without through an intermediate transfer member; and a fixing step of fixing the toner image on the recording material under heat.
2. Description of the Related Art
An image forming apparatus employing an electrophotographic method which has been widely demanded for an office use purpose and a personal use purpose, and in any one of the markets such as a graphic market and a light printing market in recent years is an image forming system excellent in quick start property and energy saving property.
Accordingly, the mainstream of, in particular, a fixing system has been shifting from a conventional hard roller system having a large heat capacity to a light-pressure fixing system such as film fixation or belt fixation having a small heat capacity from the viewpoint of a reduction in power consumption (see, for example, JP 2005-055523 A and JP 2005-056596 A).
Since such light-pressure fixing system has a small heat capacity, the system can shorten a time period required for the temperature of the system to reach a fixation set temperature (which may hereinafter be referred to as “adjustment temperature”), and is excellent in quick start property. In addition, the system has the following advantage: a fixing unit itself can be reduced in size and weight because the system does not use a thick metal part or multiple heaters unlike a conventional hard roller system.
On the other hand, however, a light-pressure fixing system shows a larger reduction in temperature of the surface of a fixing member upon continuous copying than that in the case of a conventional hard roller system owing to a reduction in heat capacity. In addition, the light-pressure fixing system is apt to reduce the pressure of toner to be applied to a recording material, so a fixing failure is apt to occur.
In contrast, in, for example, film fixation out of the light-pressure fixing systems, a fixing member that sufficiently fixes a toner image on a recording material for preventing a reduction in temperature at a region where the fixing member and a pressurizing member contact with each other (which may hereinafter be referred to as “fixing nip”) has been proposed (see, for example, JP 2005-056738 A). However, such light-pressure fixing system is still apt to cause a reduction in temperature of the surface of the fixing member, and a fixation temperature distribution and a fixing pressure distribution at the fixing nip are apt to be nonuniform as compared to a conventional hard roller system. Accordingly, a fixing failure due to the reduction in temperature, or the so-called hot offset phenomenon in which toner adheres to the fixing member at a fixing nip portion having a temperature in excess of an adjustment temperature to contaminate the fixing member, and the contaminated fixing member contaminates the recording material when the fixing member contacts with the recording material again is apt to occur. Various contrivances have been made to prevent such reduction in temperature as described above, and to uniformize such fixation temperature distribution and fixing pressure distribution at a fixing nip portion as described above, but the additional improvement of the contrivances has been requested.
Therefore, each of additionally improved low-temperature fixability and a wide fixation temperature range (which may hereinafter be referred to as “fixation latitude”) is performance that has been requested of toner in order that the toner may adapt to not only a conventional hard roller system but also a light-pressure fixing system excellent in energy saving property.
In addition, additional improvements in speed and image quality have been needed in an image forming apparatus employing an electrophotographic method in recent years. However, an improvement in developing ability and an improvement in such low-temperature fixability as described above with a view to corresponding to the high-speed developing system are in a trade-off relationship. For example, in the case of toner placing priority on low-temperature fixability, the molecular weight distribution of a binder resin tends to be made small, or the softening point of the resin tends to be reduced. As a result, detrimental effects such as the deterioration of the toner and the contamination of a developing member at the time of high-speed development are apt to occur. In contrast, in the case of toner placing priority on developing ability, the molecular weight distribution of a binder resin tends to be made large, or the softening point of the binder resin tends to be increased. As a result, the low-temperature fixability of the toner deteriorates, so it becomes difficult to achieve an image forming system excellent in energy saving property.
In view of the foregoing, a high level of compatibility between fixing ability and developing ability has been requested of toner adaptable to a high-speed developing system and a light-pressure fixing system in order to correspond to needs in the market.
Various contrivances have been conventionally made to provide toner for achieving compatibility between fixing ability and developing ability. For example, a large number of toners each using a low-softening-point resin and a high-softening-point resin in combination and each taking advantage of the properties of the respective resins have been proposed. Those toners each aim to achieve compatibility between fixing ability and developing ability while securing a fixation latitude through an improvement in low-temperature fixability of the low-softening-point resin and an improvement in hot offset property of the high-softening-point resin and keeping a balance between the improvements.
Of such proposals, some proposals relate to toners each using two or more kinds of resins in combination and each having the so-called sea-island structure in which a low-softening-point resin is included in the structure of a high-softening-point resin (see, for example, JP 2002-214833 A and JP 2002-244338 A). Those toners are each excellent in that the elution of the low-softening-point resin is controlled, and a fixation latitude is secured. However, an additional improvement in low-temperature fixability is requested in order that each of the toners may adapt to such light-pressure fixing system as described above.
In addition, another proposal concerning toner using a low-softening-point resin and a high-softening-point resin in combination is as follows: the combined use of two or more kinds of resins compatibility between which is good satisfies the low-temperature fixability and storage stability of toner (see, for example, JP 2000-275908 A and JP 2004-085605 A). However, such proposal is still insufficient in terms of the securement of a fixation latitude in the above-mentioned light-pressure fixing system and an improvement in developing ability in a high-speed developing system.
Accordingly, at present, there still remains a problem concerning a high level of compatibility between fixing ability and developing ability.